Create a wispy, foamy top? Like Hot Cocoa?
I made a quick render of what you described. Here I essentially have a whole lot of bubbles on the surface of a rectangle, which is basically what the froth is. You could increase the number, size, etc of the bubbles easily, my computer just can't handle more than this :)



To make the large bubbles, I made a single sphere, and colored it a semi-transparent brown color. Next I duplicated it a few times and used the random transform tool object>transform>randomize transform as seen here

Next I made one small sphere into a group and used the particle tool (great post on that here) to scatter them randomly across the surface. The post I mentioned show how to use the options to make the bubbles smaller, make more, etc.

I recognize that this doesn't look exactly like hot cocoa, but if you just color the bubbles a bit differently, adding some more white and a bit of dark brown it'll be pretty close.